Most of the LCD light valve projection TV systems currently under development can be broken into two broad classes: 1) those using multiple exit pupils where a separate lens is used to project each of three different color LCDs, and 2) single exit pupil systems where all three different color LCDs are projected through the same lens.
In either case, the system can be of the transmissive type where light comes in from behind the liquid crystal panel and is modulated as it passes through the panel or of the reflective type where light enters through the front of the panel and is reflected back out towards the screen after having been modulated.
Examples of such television systems and of lenses which can be used with such systems can be found in, for example, Taylor, U.S. Pat. No. 4,189,211, Gagnon et al., U.S. Pat. No. 4,425,028, Gagnon, U.S. Pat. No. 4,461,542, Ledebuhr, U.S. Pat. No. 4,826,311, Minefuji, U.S. Pat. No. 4,913,540, EPO Patent Publication No. 311,116, and Russian Patent Publication No. 1,007,068. A lens described as being "retrotelecentric" is disclosed in Ikemori, U.S. Pat. No. 3,947,094.
Systems having a single exit pupil solve the problem of color shift in the projected image and allow for a simpler design of the projection screen in that the screen does not need to perform mixing of the colors from the three lens systems. However, the projection lens must have a large separation between the LCD panels and the lens to accommodate the filters and beamsplitters used to combine the light from the different LCDs into a common beam for projection onto the screen by the lens.
For rear projection applications, it is desirable to have as small an overall package size (set size) as possible. In terms of the optics, this means that the imaging conjugates should be made as small as possible while still maintaining a large image size. This, in turn, means that the projection lens must have a large field of view. Additionally, it is desirable to use lenses having a small physical size so as to reduce the sizes of the folding mirrors placed between the lens and the screen. Lenses having small physical sizes also help to further reduce the size of the overall TV package.
The illumination of the LCD panel plays a very important role in the performance of an LCD projection TV. In particular, it is very important to match the location and size of the exit pupil of the illumination system with the entrance pupil of the lens to obtain a bright, uniformly-illuminated TV image. Since illumination optics generally work best when the exit pupil is located a long distance from the light source, it is desirable to use a projection lens with a long entrance pupil distance.
The lens described below addresses all the above mentioned requirements and at the same time provides a high level of image quality and, in particular, a high level of correction of both lateral and longitudinal chromatic aberrations.